1. Field of the Invention
The invention relates to separation and purification, particularly separation of crystals of an aromatic organic compound, particularly para-nitrochlorobenzene, from a mother liquor.
2. Description of the Prior Art
In the commercial purification of aromatic organic compounds by crystallization, mechanical techniques such as filtration, decantation or centrifuge are used to remove as much as possible of the adhering mother liquor from the crystals. Residual mother liquor after centrifuging is ordinarily in the range of about 2-5% or more of the weight of crystals. Large uniform crystals from low-viscosity mother liquors will retain a minimum proportion of mother liquor. Non-uniform small crystals from viscous solutions will retain a considerably larger proportion. It is common practice to wash crystals on a centrifuge or to filter with a fresh solvent; and the use of counter current washing in multiple stages is known to reduce the loss of crystals by solution in the solvent. The use of a solvent requires separate steps to remove residual solvent from the product such as by drying and some means of solvent recovery. Batch press squeeze purification has been employed commercially but its lack of continuity of operation has been a serious handicap. U.S. Pat. No. 3,113,161 teaches the use of a screw press purifier in the production of naphthalene.
In the commercial production of nitrochlorobenzene, the mother liquor ordinarily contains nitrochlorobenzene isomers, trace organic impurities and trace amounts of water. This mother liquor will have ordinarily been pretreated to remove most inorganic acids, monochlorobenzene and water prior to the purification contemplated herein. Any improved method of purifying would be a significant advance in the art and is an object of this invention.